When it is down after midnight to around 6am then it could be overnight maintenance taking place as per this message at the top of the page:
https://helpforum.sky.com/t5/Broadband/Important-Notice-Scheduled-Broadband-Maintenance/m-p/5151847#...
If it still down later then the first thing to check is this link to see if there are any outages in your area, it doesn't always pick up issues straight away so it might be worth trying again later if this is the case:
https://www.sky.com/servicechecker
If your ONT shows a red light then have a read of this:
https://helpforum.sky.com/t5/Broadband-Talk/Can-t-connect-to-broadband-and-have-a-red-light-on-your-...
If there isn't try running the line test here:
https://www.sky.com/help/articles/broadband-diagnostic-start
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